[en] By the method of deuteroexchange in the process of styrene hydrocarboxylation, catalyzed by the complex PdCl₂(Ph₃P)₂ (11O deg, carbon oxide pressure 2.0 MPa) it has been ascertained that terminal protons of unreacted substrate are to a noticeable extent replaced by deuterium, whereas no exchange of protons in α-position as to phenyl group is observed. Decrease of pressure and introduction of SnCl₂ leads to proton exchange in both carbon atoms of vinyl group of styrene. The results obtained are interpreted as regards hydride mechanism of the process